Sensitized ammonium nitrate explosive and method of manufacture



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This invention relates to a sensitized ammonium nitrate explosive and to a method of manufacturing the same. In one aspect it relates to sensitizing an ammonium nitrate-normally liquid hydrocarbon mixture with deposited crystals of ammonium nitrate so that the explosive produced will detonate completely when an ordinary dyna mite blasting cap is exploded therein. In another aspect it relates to an explosive consisting essentially of a mixture of 90 to 95 parts by weight of ammonium nitrate and 10 to 5 parts of normally liquid hydrocarbon, said mixture being sensitized by drying thereon from 15 to 3 0 parts of a substantially saturated aqueous solution of ammonium nitrate having a temperature between 185 and 200 F.

In the prior art of making and using explosives, it has been desirable to use as large a percentage of ammonium nitrate of fertilizer grade as possible, because it is about the cheapest available source of explosive power, is easy to handle, and stable in storage. In order to make it explode at all it has been necessary to add organic material to the ammonium nitrate, and in order to explode it by the usual blasting cap it has been necessary to have about nitroglycerin in the explosive composition. Nitroglycerin is both expensive and dangerous to handle, and tends to segregate on storage or freeze in cold weather, and thereby makes the explosive unduly sensitive in portions and causes incomplete detonation in other portions. Attempts have been made in the prior art to sensitize ammonium nitrate by mixing crystals of the same with aqueous solutions of metal nitrate salts at room temperature, and then mixing the same with organic material, but metal nitrates are relatively expensive and difficult to obtain, and the concentration of metal nitrates is relatively low at room temperatures, even in saturated solutions.

It has now been found that mixtures of fertilizer grade ammonium nitrate with a normally liquid hydrocarbon, such as diesel fuel, can be sufiiciently sensitized to complete detonation by an ordinary blasting cap by simply adding a substantially saturated aqueous solution of fertilizer grade ammonium nitrate at a temperature above 150 F., preferably between 185 and 200 F,, and more preferably between 190 and 195 F., and then drying the mixture. The process also works with pure ammonium nitrate, but fertilizer grade ammonium nitrate containing the usual minor amounts of impurities and/or diatomaceous or other common coating material in the usual amounts used in fertilizer grade ammonium nitrate as a coating to minimize sticking of crystals is still sufficiently pure for the practice of this invention, being used in the specific examples below, and is regarded as covered by the terms ammonium nitrate and consisting essentially of ammonium nitrate, as used in the claims. The fertilizer grade ammonium nitrate generally comes in the form of small prills, of not more than 3% by weight diatomaceous earth coated crystals about inch in diameter, and it has been found that grinding it to different particle sizes does not measurably change the sensitivity of this mixture with diesel fuel. The addition of up to 7% of various materials, such as commercial oil additives, coal, sulfur, inorganic nitrates, and metal powders did not increase the sensitivity of the fertilizer grade ammonium nitrate diesel fuel mixture. Storage for two months in polyethylene bags had no efiect on sensitivity.

The normally liquid hydrocarbon used in the present invention may range anywhere from pentane to heavy mineral oil of the type used internally as a laxative. While liquefied propane or butane could be employed with some results, even gasoline containing propane or butane is not considered entirely suitable, as it is not desirable to have explosive hydrocarbon vapors coming off the explosive at atmospheric temperatures during transit, or use, and so for practical purposes the normally liquid hydrocarbon is selected from the group consisting of diesel fuel, kerosene, gas oil, fuel oil, and mineral oil. Diesel fuel is preferred. While some results can be obtained outside the following range, better results are obtained by staying near the approximate stoichiometric proportions of 7 parts by weight diesel fuel to 93 parts ammonium nitrate in the original mixture, the best results being obtained with to 95 parts ammonium nitrate and 10 to 5 parts of normally liquid hydrocarbon in the original mixture. In the process of sensitizing this original mixture from 15 to 30 parts of ammonium nitrate are deposited in crystals thereon, from a substantially saturated aqueous solution of ammonium nitrate at a temperature above F., preferably between and 200 F., and more preferably between and F. Preferably, also, the solution should be maintained at this temperature for from one to five minutes. One convenient method of preparation is to use a slight excess of water, then evaporate the solution with stirring until the appearance of solid phase. From 15 to 30 percent and preferably 25 percent by weight of this solution is used, based upon the ammonium nitrate-hydrocarbon explosive.

The aqueous ammonium nitrate solution is added to the explosive composition, and the whole is thoroughly mixed. The mixture is then dried to a moisture content of 0.3 to 1.5 percent, preferably 0.5 percent, by methods well known to the art including desiccators, vacuum, or the circulation of warm dry gas.

The sensitivity of the final product to detonation is determined by Well known standard tests.

Example Forty grams of fertilizer grade uncoated ammonium nitrate were dissolved in 10 cc. of Water at a temperature of 190195 F. to give a saturated solution. After a few minutes, this solution was slowly added to a mixture comprising 186 grams of fertilizer grade diatomaceous earth coated ammonium nitrate prills and 14 grams of diesel oil. This final mixture was then thoroughly stirred, and finally dried in a desiccator for 24 hours.

An iron pipe 1 /2 inch ID. by 5 inches was packed with approximately 110 grams of the dried product. detonator of 50 grams of 40 percent strength ammonium gelatin was separated from the charge by 16 standard cellulose acetate cards, each 0.010 inch thick. Upon detonation, the pipe was completely shattered. A control of the same ammonium nitrate-diesel oil mixture, untreated with hot aqueous ammonium nitrate solution, failed to detonate when subjected to this same test also employing 16 cards.

'In preparing the saturated aqueous solution, one could just as well use said coated fertilizer prills as using said uncoated fertilizer grade ammonium nitrate above, as the coating is water insoluble and would be segregated during the solution of the ammonium nitrate, which would not be delayed by the coating. This would be especially useful in remote areas of the United States, such as Alaska, where only the coated fertilizer would be available. This is because the coating makes the prills more stable against moisture absorption and caking in the shipping sack, and the uncoated material would not be on sale in remote areas because it would tend to cake up in transit.

Except for the diatomaceous earth, wax, or clay coating, which amounts to about 3% of the weight when employed, fertilizer grade ammonium nitrate is over 97% pure.

The specific coated ammonium nitrate in the example above was Phillips 66 Ammonium Nitrate and had a typical composition analysis as follows:

This material was in the form of prills (globules formed in a shot tower) of the following cumulative sizes:

Size in mesh: Cumulative, percent 8 0.5 l 6.8 12 23.4 14 V 84.5 I 98.3 99.5

This means it was mostly about 14 mesh, and it was coated with about 3% diatomaceous earth. The diatom-aceous eai'th and other materials present were substan tially inert so far as the present process was concerned.

The Field Director of Ammunition Plants Specification for ammonium nitrate, fertilizer grade, may be summarized as requiring the nitric acid, ammonia, and clay or wax coating to comply with various Army specifications, and the composition to be:

Material (character) Maximum, Minimum,

percent percent Moisture 0. 25 Ether soluble:

Type I- Type II. Water insoluble Total nitro en Which, compared with the composition analysis of Philips 66 Ammonium Nitrate above, indicates that fertilizer grade ammonium nitrate is substantially pure so far as the purposes of the present invention are con cerned.

While a specific example has been described for illustrative purposes it should be obvious from the description that the invention is not limited thereto. The essence of the invention, or object thereof, is to provide an ammonium nitrate-hydrocarbon explosive which is relatively sensitive to detonation, and to provide a process of making the same.

Having described my invention, I claim:

1. The process of making an explosive sensitive to detonation which comprises the steps of mixing to parts by weight of ammonium nitrate with 10 to 5 parts of a. normally liquid hydrocarbon to form a substantially dry first mixture, preparing a substantially saturated aqueous hot solution of 15 to 30 parts of ammonium nitrate having a temperature between and 200 F., pouring said hot solution into said first mixture and drying the resulting second mixture with the. formation of ammonium nitrate crystals from said hot solution deposited on the surface of said first mixture whereby said second mixture is made sensitive to detonation.

2. The process of claim 1 in which the second mixture is dried to a moisture content of 0.3 to 1.5 percent by weight.

3. The method of claim 1 in which the hydrocarbon is selected from the group consisting of diesel fuel, kerosene, gas oil, fuel oil, and mineral oil, and the temperature is between 185 and 200 F.

4. The method of claim 1 in which the hydrocarbon is diesel fuel and the temperature is between and F. 7

References Cited in the file of this patent UNITED STATES PATENTS 2,063,572 Woodbury et a1. Dec. 8, 1936 2,398,071 Barab Apr. 9, 1946 2,798,801 Kiefier et al. July 9, 1957 FOREIGN PATENTS V r 152,199 Great Britain Oct. 14, 1920 

1. THE PROCESS OF MAKING AN EXPLOSIVE SENSITIVE TO DETONATION WHICH COMPRISES THE STEPS OF MIXING 90 TO 95 PARTS BY WEIGHT OF AMMONIUM NITRATE WITH 10 TO 5 PARTS OF A NORMALLY LIQUID HYDROCARBON TO FORM A SUBSTANTIALLY DRY FIRST MIXTURE, PREPARING A SUBSTANTIALLY SATURATED AQUEOUS HOT SOLUTION OF 15 TO 30 PARTS OF AMMONIUM NITRATE HAVING A TEMPERATURE BETWEEN 185 AND 200*F., POURING SAID HOT SOLUTION INTO SAID FIRST MIXTURE AND DRYING THE RESULTING SECOND MIXTURE WITH THE FORMATION OF AMMONIUM NITRATE CRYSTALS FROM SAID HOT SOLUTION DEPOSITED ON THE SURFACE OF SAID FIRST MIXTURE WHEREBY SAID SECOND MIXTURE IS MADE SENSITIVE TO DETONATION. 